I am delighted to introduce this book on the use of monads in Haskell as a way of structuring interpreters. In the early days, Haskell's most distinctive feature was lazy evaluation. Laziness forced us to take a pure approach to input/output, which meant that Haskell's I/O was initially rather weak. This weakness ultimately proved a strength, however, because it led us to the discovery that monads were not just an abstract mathematical concept, but were immediately applicable as a powerful program structuring mechanism.

Monadic programming is not just to do with input/output: it is much more powerful. That is why I am pleased to see this book, which describes in some detail how to write a language interpreter using a monadic approach.

In retrospect, the discovery of monads as a practical programming pattern is one of Haskell's most substantial contributions to the world of programming -- and it is one that you will share if you work through this book.

I am also very happy to see Haskell growing in popularity among our brothers and sisters in Eastern Europe, and in Romania in particular. Enjoy!